Bioremediation, the use of microorganisms to detoxify hazardous contaminants, has long been considered a promising method to provide economical and ecologically sound clean-up strategies. The U.S. Environmental Protection Agency (EPA) has classified bioremediation as one of the most promising and potentially economical innovative treatment technologies. Three major considerations in selection of the most appropriate strategy to implement bioremediation at a specific site include the amenability of the pollutant to biotransformation to less toxic products, the accessibility of the pollutant to microorganisms, and the optimization of biological activity. For metal contaminants that cannot be chemically degraded, microbial systems can biotransform toxic metal species to ones that are less toxic, immobilized, or are easier to recover.
Bioremediation can utilize naturally occurring microorganisms, or enhancing microbial organisms to better effect the desired detoxification process. In the latter method, intrinsic microbial activities are increased through various methods, including physiologically augmenting the capabilities of indigenous microbial consortia, enhancing indigenous consortia by supplementing with more robust microorganisms with a higher bioactivity for the contaminant, biotransforming or binding metals; manipulating the environment, for example with air sparging to maintain an aerobic environment, and other means to increase the desired metabolic degradation of organics, or the immobilization, detoxification, or binding of metals.
As bioremediation relies on microorganisms, microorganisms with biochemical and physiological characteristics suitable for the task must be identified or created. Microorganisms for bioremediation applications may be identified by isolating strains or consortia from sites that have been heavily contaminated with a given pollutant. The microbial population isolated from the contaminated site can be enriched for degraders of the given pollutant in enrichment cultures. Alternatively, strains or consortia isolated from contaminated sites or from laboratory culture collections can be screened for the desired bioremediation trait. Various methods have been developed for screening such microorganisms.
In many cases, naturally occurring organisms are not optimized for the desired bioremediation activity. For such organisms, it may be desirable to modify the inherent genetic capability in order to enhance a characteristic of interest.
Techniques are required for removing dissolved uranium from a variety of surface waters, groundwaters, and waste streams in order to prevent or remediate environmental pollution. For example, several steps in the mining and processing of uranium either generate uranium-bearing waste streams or have the potential for contaminating natural waters. Furthermore, the nuclear weapons production program in the US and elsewhere has resulted in large scale contamination of soils, sediments and waters by uranium. At the US Department of Energy waste sites, this contamination is estimated to cost billions of dollars for cleanup. The remediation of uranium-contaminated soils is likely to generate large volumes of uranium-bearing leachate. Irrigation practices may result in surface waters or groundwaters with elevated levels of uranium.
Ion-exchange resins are commonly used to remove uranium from water. However, the desirability of ion-exchange methods can be limited by the material costs, interferences by competing ions, poor extraction at low uranium concentrations, and the production of a large volume of waste when the exchange resin is disposed or the generation of a highly corrosive uranium-containing waste if the uranium is extracted from the resin. Thus, there has been interest in developing alternative mechanisms for uranium removal.
The potential for bioprocessing of uranium and other radioactive wastes and ores is receiving increased attention because microbially based processes may provide cost effective mechanisms for metal removal. Several biological methods for extracting dissolved uranium have been developed.
The finding that several microorganisms can enzymatically reduce U(VI) to U(IV) has suggested an alternative enzymatic process for removing dissolved uranium from water. Dissolved uranium is typically in the oxidized form, U(VI), which is highly soluble. In contrast, the reduced form, U(IV), is highly insoluble in most waters. Microbial U(VI) reduction converts dissolved U(VI) to an extracellular precipitate of the mineral uraninite (UO2). This reductive precipitation of uranium has been proposed to account for the accumulation of uranium in various anaerobic environments such as aquatic sediments, roll-front ore deposits, and reduction spots. The ability of microbial U(VI) reduction to precipitate U(IV) from U(VI)-amended groundwater suggested that this metabolism might be used to remove U(VI) from contaminated waters.
Cr(VI) compounds, primarily in the forms of chromate (CrO42−) and dichromate (Cr2O72−) are common environment pollutants in soils and water. These compounds have become widely distributed in the environment from their use in a variety of commercial processes, such as rust proofing, metal plating, and manufacture of dyes and inks, as well as chromite ore processing. As a result of contaminated discharges from these industrial applications and inappropriate waste disposal practices, significant amounts of chromate and dichromate have contaminated the environment. Furthermore, the nuclear weapons production program in the US and elsewhere has resulted in large scale contamination of soils, sediments and waters by chromate. At the US Department of Energy waste sites, this contamination is estimated to cost billions of dollars for cleanup. As an environmental pollutant, Cr(VI) represents a considerable health risk.
Improved compositions and methods for bioremediation of toxic metals, including uranyl and chromate are of great interest. The present invention addresses these issues.
Relevant Literature
European Organization for Research and Treatment of Cancer “Molecular Targets and Cancer Therapeutics” Conference, Philadelphia, Pa. Nov. 14-18 2005. Poster A33: Improvement of a Novel Enzyme, Using Directed Evolution, for Reductive Cancer Chemotherapy, Barak et al. Analysis of novel soluble chromate and uranyl reductases and generation of an improved enzyme using directed evolution, Barak et al.